Certain marine propulsion units, such as outboard drives and inboard/outboard stern drives, utilize a forward-neutral-reverse transmission along with twin propellers. The typical twin propeller system includes a vertical drive shaft which is operably connected to the engine and is journaled for rotation in the lower gearcase. The lower end of the drive shaft carries a pinion which drives a pair of coaxial bevel gears that are located in the lower torpedo-shaped section of the gearcase. Inner and outer propeller shafts are mounted concentrically in the lower section and each propeller shaft carries a propeller, with the propeller of the outer shaft being located forwardly of the propeller of the inner shaft.
U.S. Pat. No. 4,793,773 is directed to a twin propeller propulsion system in which both propellers are rotated at the same speed, but in opposite directions, during forward movement of the watercraft. With this system, a mechanism is provided to disconnect the outer propeller shaft when the watercraft is moved in the reverse direction. Thus, with the system shown in the aforementioned patent, both propellers are operated during forward movement of the watercraft, but only the inner propeller shaft and the rear propeller are operated during reverse movement.
Co-pending U.S. Pat. application Ser. No. 08/719,633, filed Sep. 25, 1996, now U.S. Pat. No. 5,766,047 is directed to a twin propeller marine propulsion system in which, during forward movement of the watercraft, only one of the propellers is driven at low engine speed and the second propeller is driven when the engine speed reaches a pre-selected elevated value.
In accordance with the construction of the aforementioned patent application, a sliding clutch mechanism having forward neutral and reverse positions is employed to selectively engage the inner propeller shaft with the bevel gears to thereby rotate the inner propeller shaft and the rear propeller in both the forward and reverse directions. In addition, a hydraulically operated multiple disc clutch located in the lower torpedo section is employed to selectively cause engagement of one of the bevel gears with the outer propeller shaft when the engine speed reaches a pre-selected elevated value, normally in the range of 3,500 rpm to 7,000 rpm, to thereby cause the second or forward propeller to rotate in the opposite direction from the rear propeller. With this construction, only the rear propeller is driven at low forward speeds, while at high forward speeds both propellers are driven.
As described in the aforementioned patent application, the multiple disc clutch is moved to the engaged position at the pre-selected elevated engine speed by supplying pressurized fluid to a piston which engages the multiple clutch discs and moves the discs to a contacting or driving position. With this construction, only a single propeller is operable at low speeds, and once the pre-selected elevated engine speed has been achieved, the second propeller is then driven, resulting in a significant improvement in acceleration of the watercraft when getting on plane.